The present invention relates to an infant care warmer adapted to provide heat for the warming of an infant and, more particularly, to a method of controlling the intensity of a heater used in an infant warmer.
In the treatment of infants, and particularly those born prematurely, it is necessary to provide heat to the infant during the treatment of such infants and to minimize the heat loss of the infant. Accordingly, a common apparatus for providing such heat is an infant warmer. In general, the infant warmer comprises a flat, planar surface on which the infant rests while some procedure is being carried out on the infant. There are normally protective guards that surround the infant to keep the infant contained within the apparatus and there is an overhead heater that directs radiant energy in the infrared spectrum toward the infant to impinge upon that infant to provide warmth.
An infant warmer is shown and described in U.S. Pat. No. 5,474,517 of Falk and is an example of the type of infant care apparatus that is used to provide warmth to an infant while carrying our some procedure on that infant
The are, of course, numerous methods and systems to control that heater, and currently, many of such warmers include an infant temperature sensor that is affixed to the skin of the infant and thus provides an input to the heater controller indicative of that infant skin temperature. In addition, there is also an input to the heater controller that can be set by the user depending on the desired temperature that the user wants to establish for the infant.
Given those inputs, the heater controller acts to counter the change in environmental temperature, and changes in the infant temperature to control the heater output to maintain the infant as closely as possible to that desired temperature set by the user.
One system used to control a radiant heater is shown and described in U.S. Pat. No. 5,162,038 of Wilker. In that patent, the control system uses a difference between the patient and the set point temperature to operate a proportional control loop and uses a proportional control technique to adjust the power to the heater. Thus, with proportional control, the power signal to the heater is based upon the difference in temperature and the same difference in temperature will result in the same adjustment to power to the heater. Accordingly, the control system of Wilker produces a predictable power for controlling the heater and respond directly and predictively to any change in temperature.
Accordingly, the present invention relates to a method and system for controlling the radiant heater used in connection with an infant warmer. The present heater control system is carried out principally in software and provides a control of the radiant heater that is both fast responding and which causes the infant temperature to accurately trace the set temperature established by the user.
In present method and system, the system basically free runs and does not provide a predictable power change to the heater for any specific temperature difference between the patient and set temperature. Thus, the present system and method is not a proportional control but seeks and finds its own level of power that is not governed by any specific temperature difference. In effect, the system and method of the present invention will adjust the heater power and the resulting heater power adjustment may be different for a variety of conditions.
In carrying out the present invention, use is still made of the patient skin temperature and which is sensed by means such as a thermistor to convert that temperature into an analog signal. In addition, use is also made of a signal that is representative of the desired or set temperature inputted by the user. Initially, the present method take the signals representative of the patient skin temperature and the set temperature and performs a subtraction function to subtract the patient temperature from the set point temperature to arrive at a differential signal identified as the Patient Temperature Gradient (PTG).
The present control system thus uses that PTG to directly control the power to the heater by means of a controller that basically supplies the power to the heater and that controller thus directly modifies the power to the heater depending upon the sign and the magnitude of the PTG. The heater has, of course, a known full power rating and the settings are based upon that power. In actual operation, the magnitude and sign of the PTG is sampled at predetermined time intervals, generally every 30 seconds. As each sample is provided to the controller, power to the heater is controlled as will be explained.
The heater is initially started as a reduced power, preferably at about 50% power on power up and the predetermined timed samples of PTG affect the power to the heater as follows:
Less than xe2x88x920.3 C., the heater is off
xe2x88x920.3 to xe2x88x920.15 C.; subtract 5% from the last heater power
xe2x88x920.15 to 0.05 C.; use last heater power
0.05 to 0.15 C.; add 1% to the last heater power
0.15 to 0.25 C.; Add 5% to the last heater power
0.25 to 0.35 C.; add 10% to the last heater power
0.35-0.50 C.; add 15% to the last heater power
If PTG is greater than 0.5 C., the heater is on full power.
As can be seen, there may be an initial step of determining whether the PTG is within certain predetermined limits described above and, if the PTG is less than the xe2x88x920.3 PTG, the system may proceed directly to the controller to set the power to the heater at 0 percent. Likewise, at that initial determination, if the PTG is greater than 0.5 C., the power to the heater can be directly set by the controller to 100% power to that heater.
Thus, as can be seen, by directly utilizing the PTG, the controller can make an immediate adjustment to the heater power in order to track the desired set point inputted by the user. In each instance, the percentage of power referenced is based on a percent of full power to the heater, thus each incremental changes made by the controller to the heater is a fixed value.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention.